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Together We Stand June 20, 2007

Posted by gordonwatts in physics.

Listening to a Higgs talk yesterday talking about prospects for a Standard Model Higgs search in 4 fb-1 and 8 fb-1 of data (2008 and 2009 results) one realizes that neither experiment can really do the job alone. Together, the two experiments can do a lot and cover a rather broad mass range. But apart neither of us are going to be able to do all that much. The Higgs is just too hard to find!

Combining results is difficult. It is difficult because of errors. When DZERO says that it has a 20% error due to Jet Energy Scale, is that really the same error that CDF reports as its Jet Energy Scale? In all likelihood, the answer is no. To do the combination properly is key to get these sorts of thing right and clear. This will require quite a bit of collaboration between the two experiments. Together, we have a chance of ruling out large stretches of the low mass range of the Standard Model Higgs: exactly the area the LHC has the most difficulty with.

I can’t wait.



1. Aaron F. - June 21, 2007

Interesting! So do you think it’s likely that when the LHC comes online, the traditional D0 / CDF rivalry will be replaced by a system of data sharing and collaboration? And if that happened, who would take on the independent-verification role that CDF and D0 currently play for each other? Your remark about the LHC’s trouble at low energies seems to imply that the LHC couldn’t do it…

2. gordonwatts - June 21, 2007

I don’t think the competitive spirit between CDF and D0 will ever go away. Not only is it healthy for exactly the reasons you mention (i.e. cross checks) but it has been with us for more than 20 years. You can’t just change things with a snap of your fingers.

And when I say work together I don’t mean actually analyze each others data. I mean that we will have to combine results.

The LHC can certianly do Higgs at low mass. However, it is hard, and it will take a while. A high mass higgs, for example, will probably be seen in about a year or less of data taking. Certian low mass regions may take a number of years before they are visible. So if the Tevatron can rule out some low mass Higgs phasespace then that will be very helpful to the LHC program. But even if the Tevatron blew up today the LHC could still cover the phase space.

The basic problem here is neither experiment has the sensitivity to do this Higgs search on its own. We have to combine the results regularly to provide meaninful progress. In the past the two experiments have always tried to obtain 3 sigma or discovery on their own. This probably won’t work. We’ll have to combine two less-than-three sigma results to obtain one over 3 sigma. That will be hard. We’ve never done that before.

But hey. Challenges are what we’re here for! 🙂

3. dorigo - June 26, 2007

Hi Gordon,

I totally agree that CDF and D0 need to work together on this one thing. And in fact, I passed to one of your PhD students who’s working on the Higgs search some private code of my own recently. Some pretty good thing to improve one critical side of the analysis. My own stuff. Now it’s his too. Weird ? No, just the right thing to do for this particular particle hunt.

What I do not agree with: I hope we can exclude the low mass region at 95% CL, but even if we do, we will give no real help to LHC. Because, if they do not see a H at 145 or above, they -nor anybody else- will have no joy in combining exclusion regions with the Tevatron and conclude that the Higgs is not there. I do not believe for a minute they would throw up their hands and say “we won’t look for H at 120 since CDF and D0 ruled that out”…

BTW, any news about the MSSM H? Another reporter from an important media contacted me, I need to give him something fresh. Hahr hahr! Just kidding, put your guns down you guys! I will continue to offer the press just my own, personal opinion – that’s all I’ve got.


4. gordonwatts - June 27, 2007

Hi T,
Several things. First, passing code is fine. Much more useful, of course, is passing techniques (i.e. seminars, papers, etc.) — as then there is a line of credit and if your ideas prove useful then you’ll get proper credit. If you just do it behind the back that leaves one open for all sorts of messes.

I totally agree with your other point. If no one sees a Higgs then all parameter space will be re-scanned. Heck, as the optimal higgs mass has dropped to well below the LEP limit there have been noises made about generating low mass higgs MC samples at the Tevatron. I see no reason why the Tevatron results wouldn’t be subjected to the same going-over if the LHC sees nothing. But if the Tevatron does rule out the low mass higgs that will keep the focus on the high mass higgs. And the nice thing is that the LHC can answer that question fairly quickly.

Last paragraph: you are playing with fire. Probably best not to joke around at this point.

Cheers, G.

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